[maxkauker]

Hey there guys,
I have a homemade CNC that I built a few years ago and now one of my dad's friends would like me to build one for him as well. I am designing it all out and it is all coming together very nicely. I have one concern though, the material. My last CNC was aluminum plate based and though very rigid, was also very expensive. The CNC I'm designing is relatively small 21"x21x4" Z and though aluminum would certainly stiffen up the machine and add professionalism, I think in this case, it may be a bit much. The user would like to cut wood and maybe (maybe) aluminum.

Attachment 235256

The parts I need to cut out will be those side triangle shapes, the back gantry, and the under gantry. What would be a cost effective alternative to aluminum in this case? Some kind of hard wood or even plastic maybe?

Triangles: Roughly 14"x8"
Gantry: Roughly 6"x 29"
Gantry under Roughly: 6"x29"

For my aluminum supplier, I can get it at around $4/lb so anything cheaper than that will certainly be considered.

Thanks,
Max Kauker

[Devastator]

Bamboo? See Jerry's thread: http://www.cnczone.com/forums/cnc-wo...t-started.html

[awerby]

The only reasons you need to use aluminum are its lightness and its straightness. For the table under the gantry, you can use steel. You just need to make sure that you've got something very flat for the rails to mount to. It's best to use aluminum for the gantry itself, since extra weight would work against you, and straightness is important. At $4/lb and considering it's a small machine, it doesn't seem like a huge investment.

Wood isn't a good material for making precision machines, since it changes size and shape with humidity variations. Plastics are too flexible, and they aren't any cheaper than aluminum. MDF is sometimes used, but it's not very strong, won't hold screws and if it gets wet, it dissolves. There's no way you're going to cut aluminum effectively on a machine made out of MDF, wood, plastic, bamboo etc.

[JerryBurks]

..... There's no way you're going to cut aluminum effectively on a machine made out of MDF, wood, plastic, bamboo etc.

Nah....absolutely not true. See the link to my thread on Devastator's post. Good quality plywood including bamboo has of course about 5 times lower modulus than aluminum and requires different shape of the components, that means about 1.5 - 2 times larger cross-section of torsion boxes etc. But it also weighs only a 5th of aluminum and, given proper design you can build a machine that is as rigid or better than an aluminum machine. While wood will expand when exposed to serious humidity variations, this effect is mitigated by using plywood. On the other hand, aluminum has a much larger thermal expansion than plywood.

I can mill aluminum on my machine without any problems and some light duty work in mild steel, too. The limitation is more in the linear bearings than in the material of the frame. But if a machine is used mainly for metal you will also need proper cooling/lubrication and a work table that can handle the liquid. Metal construction is hard to beat for that purpose.


Now, the major reason for me to use bamboo was (besides the desire to experiment) the ease of fabricating and joining compared to metal. Good quality bamboo plywood is not cheap and in most locations it must be shipped in at additional cost. It may save some money over aluminum but not that much. Good quality hardwood plywood like Apply Ply may be easier to get, is cheaper and almost as good. I am also not a fan of MDF. It is not very strong and the rigidity is a fraction of plywood. You will need much more material to compensate.

[Devastator]

There's no way you're going to cut aluminum effectively on a machine made out of MDF, wood, plastic, bamboo etc.

I've seen some pretty effective aluminum machining being done by wood machines. Maybe not optimal, but effective. Several of the wood machines I have seen would no doubt be more rigid than a machine built solely with aluminum extrusions. I'm a big fan of composite wood/steel. Where wood forms the core and steel is bolted to it in strategic areas. If engineered properly, I believe that approach would exceed aluminum extrusion in rigidity.

Absolutely, steel is best followed by aluminum, but by no means can wood be disqualified in a proper design. Not a fan of MDF unless I'm building speaker cabinets.

[maxkauker]

Wow that bamboo cnc looks very well constructed. The Aluminum in total should only cost around $130 so it certainly isn't a large investment to you or I who are in this machining community. It is however an investment to someone who wants to do very mild around the house machining.
MDF 100% is out of the question. My CNC table top is MDF and I need to resurface it nearly every month just to keep a tolerable Z level. I am however curious about the field of plywoods. I have a Home Depot a mile from my house but I haven't seen too much high grade wood. Mostly really poorly constructed .5" sheets for structure of houses. Very unappealing stuff. I may have to convince him to get that extra $120 to get the aluminum construction. This whole machine should be built for under $1100 w/ aluminum.

Now I'm curious. When you say with the "right" engineering you can make wood/steel be used together, how so? Use the steel as like corner braces and stringers to brace the wood?

[dmalicky]

Baltic Birch plywood has the best mix of properties: reasonable cost, good hardness, good stiffness, excellent uniformity and availability.
As Jerry said, you mainly need larger cross-section sizes to make up for the lower modulus.
I find BB ply is a good choice for the base, legs, and gantry. The best place for aluminum is the Y car and Z car: there the space is constrained and the stresses are concentrated, so wood's lower modulus becomes a problem.

[Devastator]

Now I'm curious. When you say with the "right" engineering you can make wood/steel be used together, how so? Use the steel as like corner braces and stringers to brace the wood?

Basically bolting it together with epoxy, thus bonding all of the parts in critical areas that are subject to twisting. Making like a sandwich in critical areas. For people like myself that are bad welders it works pretty well. For instance using angle iron on the wood gantry beam bolted and epoxied will decrease the twisting you will normally see with an all wood gantry. Same goes for your X rail mounting areas. With all wood it is prone to eventual sagging unless it extremely well supported along the whole length, but if you add some structural steel you can decrease or eliminate it.

It is basically like making steel box tubing with a wood core, all bolted and epoxied together. It is very strong, but the weight can add up quickly.

[maxkauker]

Interesting. That design sounds like it would be best suited for a skateboard bearing clamped setup for the cornersor a v rail setup. I guess I could get that to work for me if I were to put the gantry rails on the top and bottom versus the setup I have with them both on the front. That would certainly give me more strength for those plunges but it would eat up some of my Z height.

Hmm. I guess that steel enforced wood approach would only be financially viable if I can get a good price on steel around here. I will keep my eyes out.

I have always wanted to use 80/20 Extrusion for one of my projects but unfortunately that stuff costs an arm and a leg to get. It isn't too expensive but shipping to Central CA is ridiculous.

[wizard]

First off wood, as in high grade plywoods, is perfectly fine for a router type machine **as long** as you don't intend to machine metallics with any kind of coolant/lube. If the machine is to be used for any significant amount of aluminum machining I really see a metallic build as required simply due to the need to flood cool. You can machine dry but don't expect the same results as you would see with a good coolant system.

The other problem is cost, Baltic birch and other plywoods are expensive and time consuming to assemble. For a machine this size you can go to your local steel dealers drops section and pick up all the steel tubing you could want cheap, possibly under a dollar a pound. If you go with square tubing much of the frame can bolt together. Yes you have to drill holes but it isn't a big deal.

The other thing that I'd consider here is a moving table design due to the machines small size and the desire to machine aluminum. You should be able to get a far more rigid machine for the amount of effort expended and thus a machine more suitable for machining metal.

You could also use aluminum tubing but that is expensive from what I've seen. Further I don't like T-slotted aluminum at all for this usage, at least not the lighter stuff available out there.

I guess what I'm trying to get at is don't dismiss steel out of hand, nothing is perfect but steel is pretty cheap compared to other build materials.

[Devastator]

Hmm. I guess that steel enforced wood approach would only be financially viable if I can get a good price on steel around here. I will keep my eyes out.

Steel is cheap and most effective. If I could do an all steel build I would but my welding skills are not so good.

[ger21]

While you can build a machine out of a variety of materials, you can't usually just swap wood or plastic for aluminum.

Different materials require different fabrication techniques, and parts need to be designed differently.

While you may be able to get away with a 1"x6" aluminum bar for a gantry, it won't work with wood or plastic. To get the same strength, it would need to be several times thicker, or a larger box beam would need to be made.

As Wizard says, if your main goal is to be machining aluminum, don't build the machine from wood.

Similar to Devastator, in my wood machine I epoxy in phenolic plates for reinforcment at bolt and nut locations, and for my rail mounting beds. without reinforcement, wood or plastics will crush when tightening bolts. I also use aluminum plates when needed for added strength.

[wizard]

Steel is cheap and most effective. If I could do an all steel build I would but my welding skills are not so good.

Steel is cheap if you also need to buy wood retail. As for welding you don't have to weld a frame built from steel. For a inexpensive build you might want to avoid welding anyways because of the post processing (stress relieve / normalize and machining). With a little effort I'd suspect that one could have a machine as rigid or maybe significantly better than one built out of T-slots. The bolts and dowels just need to be put in the right places.

All that said I'd by far prefer well designed weldments where it makes sense. The problem is preference and budget don't always coincide. In any event I just want to suggest that steel doesn't imply that you have to weld the frame together. There certainly are advantages to welding up parts of a steel structure but just as there are many ways to do a wood frame so too can you use different approaches for steel.

[Devastator]

In any event I just want to suggest that steel doesn't imply that you have to weld the frame together. There certainly are advantages to welding up parts of a steel structure but just as there are many ways to do a wood frame so too can you use different approaches for steel.

Agreed. I may have to get creative on my next build to use mostly steel and bolts/pins. Machining a ton of aluminum dry with a single flute bit takes a long time. If I use coolant I can use a three flute and go three times faster. I was cutting @135IPM with hand sprayed coolant but got tired of standing there during a multi hour run, so I hooked up coolant mist but it made too big of a mess to contain away from the wood. So I went back to single flute @45 IPM and dry cut. At least I don't have to stand there spraying the bit.